Heavy metal concentrations and arsenic speciation in animal manure composts in China

A nationwide survey of animal manure-based composts was carried out in China. Two hundred and twelve samples were collected and analyzed for the concentrations of 9 heavy metals or metalloids and arsenic (As) speciation. The concentrations of heavy metals vary widely (Zn 11.8-3692, Cu 3.6-916, Cr 0.7-6603, Ni 0.7-73, Pb 0.05-189, As 0.4-72, Co 0.1-94, Cd 0.01-8.7 and Hg 0.01-1.9 mg kg−1 dry weight). Compared with the maximum permissible values of the Chinese standard for organic fertilizers, 13.7% and 2.4% of the composts exceeded the limits for As (15 mg kg−1) and Cd (3 mg kg−1), respectively. Some samples contained very high concentrations of Zn and Cu, although no limits have been set for these two metals in China. Further analysis showed that As was present mainly as arsenate (AsⅤ), with dimethylarsenate (DMA) and monomethylarsenate (MMA) as the minor species in composts. There were significant correlations between the concentrations of Zn, Cu and As, suggesting co-contamination of these elements in some composts. Monte Carlo simulations based on the distributions of metal concentrations and application rate showed that repeated applications of compost likely increase the concentrations of Zn, Cu, Cd and Hg in soil significantly compared with the soil background levels. This study highlights a need to minimize the concentrations of Zn, Cu, Cd, Hg and As in animal manures to ensure their safe recycling to agricultural soils.